Process for electroplating chromium and electrolytes therefor

ABSTRACT

In accordance with certain of its aspects, this invention relates to novel compositions and to a process for electroplating chromium plate onto a basis metal which comprises passing current from an anode to a cathode at least a portion of which contains a conductive metal layer through an aqueous acidic chromium-plating solution substantially free of nitric acid and other oxidative catalysts containing 1. AT LEAST ONE CHROMIUM COMPOUND PROVIDING HEXAVALENT CHROMIUM IONS FOR ELECTROPLATING CHROMIUM; AND 2. CEROUS IONS IN COMBINATION WITH FLUORIDE IONS AND SULFATE IONS AS CATALYSTS; FOR A TIME SUFFICIENT TO DEPOSIT A CHROMIUM ELECTROPLATE HAVING A THICKNESS OF AT LEAST 1 X 10 4 MM.

nited States Patent [72] Inventor Edgar J. Seyb, Jr.

Oak Park, Mich.

[2 1] Appl. No. 864,170

[22] Filed Oct. 6, 1969 [45] Patented Jan. 11, 1972 [73] Assignee M & T Chemicals Inc.

New York, NY.

[54] PROCESS FOR ELECTROPLATIN G CHROMIUM AND ELECTROLYTES THEREFOR Machine Searched [5 6] References Cited UNITED STATES PATENTS 3,334,033 8/ l 967 Romanowski et al 204/51 Primary Examiner-G. L. Kaplan Attorneys-Lewis C. Brown, Kenneth G. Wheeless and Robert P. Grindle ABSTRACT: In accordance with certain of its aspects, this invention relates to novel compositions and to a process for electroplating chromium plate onto a basis metal which comprises passing current from an anode to a cathode at least a portion of which contains a conductive metal layer through an aqueous acidic chromium-plating solution substantially free of nitric acid and other oxidative catalysts containing 1. at least one chromium compound providing hexavalent chromium ions for electroplating chromium; and

2. cerous ions in combination with fluoride ions and sulfate ions as catalysts;

for a time sufficient to deposit a chromium electroplate having a thickness of at least 1X10 mm.

PROCESS FORELECTROPLATING CHROMIUM AND ELECTROLYTES THEREFOR This invention relates to novel compositions and to novel processes for the electrodeposition of chromium from an aqueous acidic chromium-plating bath.

lt is knownthat chromium may be plated onto various basis metals from aqueous bath solutions. During chromium plating using such bath compositions it has been found that the use of certain catalysts such as silicofluoride ions and-sulfate ions provide improved chromium electrodeposits especially insocalled low current density areas. The effect of such catalyst systems, however, has been found to vary with the concentration of the chromic acid and the ratio of the catalysts to the total chromic acid'content. Since the concentration of the chromium-plating bath continually varies during operation due to the removal of chromium in the form of deposited metal and due-to the addition of impurities arising from the electrodes as well as decomposition products, it has been difficult to control and maintain the proper catalyst and chromic acid concentrations throughout the life of a typical aqueous acidic chromium-electroplating bath.

it has now been found that the addition of cerous ions in combination with fluoride and sulfate catalyst ions in an aqueous acidic bath containing chromic acid results in a chromium-electroplating composition which is stable over the life of the bath and maintains the activity of the bath over its useful life by regulating the concentrations of the catalyst ions as the chromic acid concentration is depleted and in the presence of typical impurities and decomposition products which occur during the chromium-plating operation.

It is an object of this invention to provide improved electroplating compositions and processes. A further object of the invention is to provide stabilized chromium-plating bath compositions which have good throwing power and extended useful life. Other object of the invention will be apparent to those skilled in the art upon inspection of the following detailed description of the invention.

In accordance with certain of its aspects, this invention relates to novel compositions and to a process for electroplating chromium plate onto a basis metal which comprises passing current from an anode to a cathode at least a portion of which contains a conductive metal layer through an aqueous acidic chromium-plating solution substantially free of nitric acid and other oxidative catalysts containing 1. at least one chromium compound providing hexavalent chromium ions for electroplating chromium; and

2. cerous ions in combination with fluoride ions and sulfate ions as catalysts; for a time sufficient to deposit a chromium electroplate having a thickness of at least I Xl' mm.

The chromium electroplating bath compositions of the invention may contain about 100-600 g./l. of chromic acid (expressed as CrO and preferably about 200-350 g./l. of chromic acid. The fluoride ions may be added to the chromium electroplating bath in the form of compounds such as sodium silicofluoride (Na SiF fluosilicic acid (H SiF strontium silicofluoride (SrSiF ammonium silicofluoride [(Nl-l Sif magnesium silicofluoride (MgSif calcium silicofluoride (CaSiF etc. Other fluoride compounds which may be used in the bath according to the invention include sodium fluoride, potassium fluoride, calcium fluoride, hydrofluoric acid, ceric fluoride, cerous fluoride, etc. Other fluoride ions (including complex fluoride ions) which may be employed include, for example, fluoaluminates, fluoborates, fluotitanates, and fluozirconates.

The sulfate ions may be added to the chromium-electroplating bath in the form of suitable sulfate compounds such as strontium sulfate (S180 sulfuric acid (H S0 lithium sulfate (Li SO ammonium sulfate [(NH.,) 80 calcium sulfate (CaSo etc.

The chromium electroplating process may use temperatures of 30-I0 C. with a chromic acid bath containing 100-600 gJl. of chromic'acid (as CrO The ratio of chromic acid to sulfate ion (Cr0 :SO may be maintained at l00-500: l ically l50-300:l and preferably about 200:1.

Cerous ions may be added to the bath in the form of salts such as cerium carbonate, cerium fluoride, cerium sulfate, etc. When cerium fluoride (CeF is employed in combination with fluoride, fluoride-containing and/or sulfate catalysts, the chromium plating bath composition (containing -600 g./l. of Cr0 may be saturated with can, Such saturation is provided to obtain self-regulation of the catalyst content of the bath. Typically, from about I to 4 g./l. or more of CeF (or other cerous salts including complex fluoride salts) may be added to a chromium plating solution containing l00-600g./l. CrO to form a chromic acid solution which contains excess undissolved cerium fluoride (CeF Excess cerous ion concentrations derived from other cerium salts maybe utilized to suppress and further control the fluoride ion concentration.

Quite unexpectedly and of great advantage is the fact that, unlike other proposed sparingly v soluble catalyst control materials, the solubility of the cerous fluoride is virtually independent of the temperature of the solution in the range of most interest during plating when compared with other sparingly soluble fluoride compounds which have been found to have an appreciable temperature coefficient of solubility. In practice this means that when use of these materials as catalyst regulators is attempted, more of the material is dissolved at the higher (operating) temperatures and appreciable material precipitates out when the solution is allowed to cool, e.g. due to a process shutdown or routine overnight or weekend interruption.

On startup, when such solution is first reheated, the proper concentration of catalyst may not be attained for some time so that plating may not be immediately started. The remarkable constancy of solubility of the cerous fluoride as employed in the invention herein makes it possible to initiate plating as soon as the solution reaches operating temperature.

These same considerations lead to other significant practical advantages. For example, other previous so-called selfregulated baths may need extensive agitation to aid in solution of the catalytic material during reheat. The new baths employed according to the invention herein may obviate the need for excessive agitation for this purpose because the proper amount of required catalyst is already dissolved.

Although chromic acid concentration of less than 100-600 g./l. may be used, the preferred range is 100-600 g./l/, and the most preferred range is 200-400 g./l.

In general, as the chromic acid concentration increases, so does the equilibrium solubility of the fluoride ion. There are at lease two beneficial results from this fact:

1. If only small changes are to be expected in normal use,

the chromic acid:fluoride ratio remains relatively constant leading to uniformity of operation; and

2. Effective regulations in chromic acidzfluoride ratios can be obtained by changes in chromic acid concentration without corresponding modifications of fluoride concentration.

The concentration of the fluoride ion may be further suppressed by the use of an excess of cerous ion as supplied, for example, by cerous carbonate or soluble cerous oxide.

It has further been found that in order to obtain the beneficial results of the invention herein, the cerous ion-containing baths must be essentially free of substantial quantities of oxidative catalysts (such as nitric acid, perchloric acid, etc.) to prevent excessive oxidation of the cerous ion to the eerie ion on electrolysis. Because the solubility of ceric fluoride is substantially greater than the solubility of cerous fluoride, the presence of ceric ions in the bath compositions of the invention is undesirable.

This may be shown by the following example. Two beakers containing chromic acid, 250 g./l., and an excess of cerous fluoride were prepared. One was the control; the other was made 2.N in nitric acid. The solutions were heated to 55 C. and electrolyzed using lead anodes at 3.6 amperes for 5 hours.

At this time the control solution showed the slight darkening expected from the formation of trivalent chromium by electrolysis. The experimental solution containing the nitric acid was virtually black thus indicating that much trivalent chromium and tetravalent cerium had been formed. The oxidation tivity is not directly proportional to the total fluoride because the complex fluorides may modify this activity.

The unexpected stability of operation ofthe plating baths of the invention may be illustrated by the use of an aqueous bath was confirmed by analyzing the solutions for fluoride. The 5 made up originally to contain 300 g./l. chromic acid, 6.6 g./l. control solution contained 0.36 g./l. of fluoride. This was the strontium sulfate, 5.8 g./l. strontium chromate, and 3.5 cerous amount expected from the equilibrium solubility of cerous fluoride. The bath was allowed to equilibrate at 46 C. for fluoride. The experimental solution contained considerably three days with preelectrolysis of 4 amp-hr./liter (4 AH/L). more fluoride, 6.68 g./l. as might be expected since a substan- Electrolysis was continued for a further 47 days so that the tial portion of the cerous ion had been converted to the ceric total electrolysis was 561 AH/L. Analyses at the beginning and ion. end of electrolysis are shown in table ll.

According to this invention, the aqueous acid chromium plating bath compositions of the invention may be employed TABLE at temperatures of about 3070 C. to prepare improved chromium plated articles by passing current from an anode to l5 Example m Hum k '6 a basis metal cathode through said aqueous acidic chromium (4.l AH/l.) ($51 :i-i/Ll plating solution at a temperature of 3070 C. for a time sufflcient to deposit a chromium electroplate having a thickness of Crq1 300 265 at least l l0 mm. Current densities which may be employed 0 8 may be l.090 amperes per square decimeter (asd) and F 1 preferably about 3-50 asd. The bath compositions of the in- Ruuo'croflso' 309/] vention may be used with lead anodes and/or lead alloy 225 Compositions p p according to the invention Electroplating was satisfactory and of substantially uniform are characterized by their excellent stability on extended use qu amy throughout i i Plating tests completed in chromium plating bath operations. In particular, the baths Standard test stl'lps m a Hull Cell Chrolmum coveroage are characterized by excellent stability and uniformity of the was measured by Platlng at 2 amperesfor 3 minutes a! 46 fluoride: sulfate ratios in the presence of chromic acid which is T coverage resllhs m ofchromlum measured fr Om the saturated with cerous fluoride. The ratios have been found to fi current denslty end of the test panels are Show" In table remain fairly uniform over wide concentrations of chromic TABLE III The following examples are submitted for the purpose of 11- lustration only and are not to be construed as limiting the Scope of the mvenuon in any I Example No. Days AH/LITER Coverage (mm.)

The unexpectedly advantageous properties obtained by using the cerous ion in combination with fluorides (including 0 41 5| complex fluorides) may be shown by reference to the exam- 1 m3 ples summarized in table l. in each experiment, a sparingly I2 5 2 soluble fluoride salt was formed and enough of the compound 2 2;: :5 was used in the bath to provide an excess of undissolved com- 5 2| 325:0 pound. Thus, a cerous fluoride-containing precipitate I6 47 561.0 50 remained undissolved in each solution and acted as a reservoir to control the concentration ofthe catalytic ion.

In each of the bath compositions described in table I, excel- 45 Tables ll and Ill clearly demonstrate that the bath composilent chromium deposits were obtained tions ofthe invention are remarkably and unexpectedly stable TABLE I.TOTAL AMO UNT OF MATERIAL IN G./L. USED FOR MAKE-UP OF THE PLATING SOLUTION Example numbers Ingredient Cliroinic acid 250 250 250 250 375 375 250 375 Cerium (cerous) catalyst:

Cerous carbonate (pentahydrate). 15. 9 13. 4 17.0 10.4 9.86 9. 86 12.0

Cerous fluoride 3. 7O 3.

Sodium fluozircoi1ate Potassium fluotitanatc-.. Sodium fluosilicate Sodium fluoaluminate- Sodium fluoride Sodium bifiuoridc- Sulfate catalyst:

Sulfuric acid (ml./l.) Strontium sulfate Sodium sulfate Analytical results:

(a) Fuoride (total) 4.0 (b) Sulfate 1.10 (c) Total fluoride-containing anion added (moles/liter) .0. 172 (d) Soluble fluoride-containing anion found (molcs/llter) 0.052

70 As shown in examples 5 and 6 of table I, an excess of cerous ion may be utilized to reduce or suppress the concentration of the fluoride ion as desired. Table I also illustrates that use of the complex fluoride provides a greater total fluoride content than the simple fluoride. It is to be noted that the catalytic acover long periods of use and show that the oxidation of cerous ion to ceric ion apparently does not occur even after electrolysis for extended time periods as long as the bath is free of nitric ion or other catalytic oxidative ions. In addition, the results confirm the uniformity of electrodeposition characteristics and prove that no substantial oxidation of cerous ion occurs, since oxidation of cerous ion to ceric ion would cause more undissolved cerous fluoride to go into solution and thus result in higher fluoride concentrations than were found on analysis.

Although this invention has been illustrated by reference to specific embodiments, modifications thereof which are clearly within the scope of the invention will be apparent to those skilled in the art.

I claim:

1. A process for electroplating chromium plate onto a basis metal which comprises passing current from an anode to a cathode at least a portion of which contains a conductive metal layer through an aqueous acidic chromium plating solution substantially free of nitric acid or other oxidative catalyst containing 1. at least one chromium compound providing hexavalent chromium ions for electroplating chromium; and

2. cerous ions in combination with fluoride ions or fluoridecontaining complex ions and sulfate ions as catalysts; for a time sufficient to deposit a chromium electroplate having a thickness of at least 1 X 2. A process for electroplating chromium plate onto a basis metal as claimed in claim 1 wherein the aqueous acidic chromium-plating solution contains about 100-600 g./l. of chromic acid.

3. A process for electroplating chromium plate onto a basis metal as claimed in claim 1 wherein the electroplating process is carried out at 3070 C.

4. A process for electroplating chromium plate onto a basis metal as claimed in claim 1 wherein the aqueous acidic chromium plating solution contains about 100-600 g./l. of chromic acid and is saturated with a cerous salt at 30-70 C.

5. A process for electroplating chromium plate onto a basis metal as claimed in claim 1 wherein the aqueous acidic chromium-plating solution contains about 100-600 g./l. of chromic acid and is saturated with CeF at 3070 C.

6. A process for electroplating chromium plant onto a basis metal as claimed in claim 1 wherein the aqueous acidic chromic acid and l4 g./l. of CeF 7. A process for electroplating chromium plate onto a basis metal as claimed in claim 1 wherein the aqueous acidic chromium-plating solution contains about 200-400 g./l. of chromic acid, about l-4 g./l. of cerous salt, and wherein the chromic acid to sulfate ion ratio is l00-550: l.

8. A composition for electroplating chromium plate onto a basis metal which is substantially free of oxidative catalyst which comprises 1. at least one chromium compound providing hexavalent chromium ions on aqueous media for electroplating chromium; and

2. cerous ions in combination with fluoride ions and sulfate ions as catalysts.

9. A composition for electroplating chromium plate onto a basis metal which is substantially free of oxidative catalyst as claimed in claim 8 wherein the concentration of chromic acid is about -600 g./l.

10. A composition for electroplating chromium plate onto a basis metal which is substantially free ofoxidative catalyst as claimed in claim 8 wherein the concentration of chromic acid is about 200-350 g./l. and the composition for electroplating is saturated with a cerous salt at 3070 C.

11. A composition for electroplating chromium plate onto a basis metal which is substantially free of oxidative catalyst as claimed in claim 8 wherein the concentration of chromic acid is about 200-350 g./l. and the composition for electroplating is saturated with CeF at 3070 C.

12. A composition for electroplating chromium plate onto a basis metal which is substantially free of oxidative catalyst as claimed in claim 8 wherein the concentration of chromic acid is 200-350 g./l. and the composition for electroplating contains 1-4 g./l. ofCeF 13. A composition for electroplating chromium plate onto a basis metal which is substantially free of oxidative catalyst as claimed in claim 8 wherein the concentration of chromic acid is about 200-400 g./l., the chromic acid to sulfate ion ratio is l0O500:l and the composition for electroplating contains about l4 g./l. of cerous salt. 

1. AT LEAST ONE CHROMIUM COMPOUND PROVIDING HEXAVALENT CHROMIUM FOR ELECTROPLATING CHROMIUM; AND
 2. CEROUS IONS IN COMBINATION WITH FLUORIDE IONS AND SULFATE IONS AS CATALYSTS; FOR A TIME SUFFICIENT TO DEPOSIT A CHROMIUM ELECTROPLATE HAVING A THICKNESS OF AT LEAST 1X10**-4 MM.
 2. cerous ions in combination with fluoride ions or fluoride-containing complex ions and sulfate ions as catalysts; for a time sufficient to deposit a chromium electroplate having a thickness of at least 1 X 10 4 mm.
 2. A process for electroplating chromium plate onto a basis metal as claimed in claim 1 wherein the aqueous acidic chromium-plating solution contains about 100-600 g./l. of chromic acid.
 2. cerous ions in combination with fluoride ions and sulfate ions as catalysts.
 3. A process for electroplating chromium plate onto a basis metal as claimed in claim 1 wherein the electroplating process is carried out at 30*-70* C.
 4. A process for electroplating chromium plate onto a basis metal as claimed in claim 1 wherein the aqueous acidic chromium plating solution contains about 100-600 g./l. of chromic acid and is saturated with a cerous salt at 30*-70* C.
 5. A process for electroplating chromium plate onto a basis metal as claimed in claim 1 wherein the aqueous acidic chromium-plating solution contains about 100-600 g./l. of chromic acid and is saturated with CeF3at 30*-70* C.
 6. A process for electroplating chromium plant onto a basis metal as claimed in claim 1 wherein the aqueous acidic chromium-plating solution contains about 200-350 g./l. of chromic acid and 1-4 g./l. of CeF3.
 7. A process for electroplating chromium plate onto a basis metal as claimed in claim 1 wherein the aqueous acidic chromium-plating solution contains about 200-400 g./l. of chromic acid, about 1-4 g./l. of cerous salt, and wherein the chromic acid to sulfate ion ratio is 100-550:1.
 8. A composition for electroplating chromium plate onto a basis metal which is substantially free of oxidative catalyst which comprises
 9. A composition for electroplating chromium plate onto a basis metal which is substantially free of oxidative catalyst as claimed in claim 8 wherein the concentration of chromic acid is about 100-600 g./l.
 10. A composition for electroplating chromium plate onto a basis metal which is substantially free of oxidative catalyst as claimed in claim 8 wherein the concentration of chromic acid is about 200-350 g./l. and the composition for electroplating is saturated with a cerous salt at 30*-70* C.
 11. A composition for electroplating chromium plate onto a basis metal which is substantially free of oxidative catalyst as claimed in claim 8 wherein the concentration of chromic acid is about 200-350 g./l. and the composition for electroplating is saturated with CeF3 at 30*-70* C.
 12. A composition for electroplating chromium plate onto a basis metal which is substantially free of oxidative catalyst as claimed in claim 8 wherein the concentration of chromic acid is 200-350 g./l. and the composition for electroplating contains 1-4 g./l. of CeF3 .
 13. A composition for electroplating chromium plate onto a basis metal which is substantially free of oxidative catalyst as claimed in claim 8 wherein the concentration of chromic acid is about 200-400 g./l., the chromic acid to sulfate ion ratio is 100-500:1 and the composition for electroplating contains about 1-4 g./l. of cerous salt. 